This invention relates generally to internal combustion engines and control thereof and more particularly to a control method for the idling speed of an engine by feedbacking the actual speed to an objective speed.
Method of this character of controlling the idling speed of an engine are known, for example, as disclosed in Japanese Laid-Open Patent Publication No. 155239/1983. In such a method, there is a step of transition to feedback control of an idling speed control valve serving as a valve means for regulating the intake air quantity of the engine in accordance with the load and operational state of the engine. In this step the idling state is detected, and, when it is determined that the engine is in a high-speed state, the idling speed control valve is controlled to a specific initial control value. Then, after the elapse of a predetermined time, the speed is gradually brought at a specific variation rate into convergence with a learned opening degree of the idling speed control valve learned during the preceding feedback control.
By this control method, since the correction quantity of the control value of the idling speed control valve is constant, there arises problems such as a temporary drop in the engine speed at the time of transition to idling state or poor convergency toward the idling speed. That is, if the correction quantity of the control value of the idling speed control valve is set at a small quantity, the drop in the engine speed will become large at the time of transition of the engine speed from the high-speed region to the idling state. If this correction quantity is set at a large quantity, the convergency toward the idling speed will become poor at the time of transition of the engine speed from the low speed region to the idling state.
Accordingly, one corrective measure which might appear to be feasible is to set the correction quantity of the valve control value, at the time of transition to the idling state, as a function of the engine speed at that time. In this case, however, if the engine speed is high at the moment of transition to the idling state, the correction quantity of the valve control value will become large, and the reduction of the valve control value from the initial control value to the learned value will require much time, whereby the convergency toward the idling speed will become poor.